This invention relates to spread spectrum communications systems and more particularly to systems using matched filters for fast acquisition in burst mode spread spectrum communications and using a pilot carrier to control the receiver downconversion.
Spread spectrum communication systems are extremely useful for rejecting intentional or unintentional interference, to lower the probability of detection of a transmission, for combating multipath transmission problems such as echoes, and to provide concurrent multiple access communications in a spectrum reuse mode. These advantages are described, for example, in the text "Spread Spectrum Communications" edited by Cook and Ellersick, published by IEEE Press, 1983, and in the article "Theory of Spread Spectrum Communications--Editorial" by Pickholtz et al., printed in IEEE Transactions on Communication, Volume COM-30, pages 855-884, May 1982. In the context of a system for transmitting data between two earth stations via a satellite transponder, spread spectrum transmission reduces the vulnerability to interference from both terrestrial sources and from other satellite-related sources. Such an arrangement also provides reuse of the same frequency spectrum by a plurality of users in a code division multiple access (CDMA) mode. In such a system, the quality of the signal received at any receiver as measured by the bit error rate (BER) depends upon the number of transmitters simultaneously using the spectrum. The type of interference caused by simultaneous use of the same frequency spectrum is known as co-user interference. The number of receivers simultaneously receiving signals in the common portion of the frequency spectrum or in the common bandwidth is not a factor in establishing the interference. If all the potential users of a particular bandwidth transmit information continuously to their respective receivers, then the number of actual users is the same as the number of potential users who are actually active. Many potential users do not transmit data continuously, but instead have a limited amount of data to communicate, which can be transmitted by means of occasional bursts of data. For example, the amount of data to be transmitted by a particular transmitter might be sufficient to warrant transmission to its receivers only 10% of the time, and during the remaining 90% of the time it has no data to send. Since the bit error rate of signals transmitted in a common bandwidth depends upon the number of transmitters simultaneously occupying the bandwidth, the number of potential users actually assigned to that bandwidth may far exceed the number that could actually use the bandwidth if the transmissions were simultaneous. For example, one thousand potential transmitters might be assigned to a common bandwidth based upon the expectancy of an average 10% use. This means that out of 1,000 potential transmitters, on the average only 100 are actually transmitting at any one time. A 10% use or even less may be anticipated, for example, if the users are retail merchants who occasionally wish to use the system to verify the legitimacy of credit card purchases. This percentage of use may be termed a "duty cycle", corresponding to the ratio of transmitting to nontransmitting time in radar terminology.
The very large ratio of potential users to actual users which appears to be possible for low duty cycles is vastly reduced if the intended receivers of the transmissions cannot quickly lock onto the desired transmission. For example, in the case in which a potential user actually uses the assigned bandwidth 10% of the time, he might be active for an interval such as 0.5 seconds during which data is transmitted, and then his transmitter would be quiescent for an average of 4.5 seconds, corresponding to a duty cycle of 0.5 seconds/5 seconds of 10%. If the spread spectrum receiver to which the transmission is made requires 1 second to acquire or lock onto the spread spectrum transmission to be received, the transmitter must be active for 1 second in addition to the 0.5 second data transmission time. Consequently, the transmitter would actually be using the common bandwidth for 1.5 seconds out of every 5 seconds, corresponding to a 30% duty cycle or activity rate rather than 10%. Rather than 1,000 potential users assigned to the common bandwidth as in the case of 10% duty cycle, the 30% duty cycle causes a reduction in the number of potential users to 333 for the assumed BER. Thus, the long acquisition time can create a substantial reduction in the usefulness of valuable electromagnetic spectrum.
Conventional spread spectrum receivers acquire lock by a bootstrap procedure. In this procedure, a local clock derived from an oscillator selected to be close to the chip rate of the received signal clocks a pseudorandom sequence (PRS) generator which generates a PRS signal corresponding to the PRS by which the transmission to be received was encoded. The received signal is downconverted to IF by a local oscillator, and the IF signal so generated is mixed with or multiplied by the PRS sequence. Because of slight difference between the chip rate of the received signal and the chip rate of the local PRS signal, the two sequences drift in phase relative to each other and eventually are in phase coincidence. At the moment phase coincidence of the received and locally generated PRS signals occurs, the output of the multiplier will produce pure IF carrier signal. This IF carrier signal is applied to a local oscillator controlling phase lock loop (PLL) which will acquire phase lock within a certain time after the two pseudorandom sequences come into phase coherence. When locked, the phase lock loop forces the local oscillator to a frequency equal to the IF frequency, so that the output of the mixer is the PRS encoded data. Once the PRS encoded data is generated, a second PLL responds to the chip rate signal to produce a continuous chip rate clock. When locked, this output clock is substituted for the locally generated chip rate clock to drive the local PRS generator, whereupon acquisition is complete. Such a system is described in U.S. patent application Ser. No. 513,737 filed by Jul. 14, 1983 in the name of Mangulis et al. This type of bootstrap acquisition procedure may be relatively slow. Each of the two PLL's has its own lock up time, and their locking must occur sequentially. The time required for the locally generated and incoming or received PRS signals to come into phase coincidence depends upon the difference between their chip frequencies. If the locally generated and received PRS clock rates are very close to each other, only a small change in phase occurs per unit time, so that phase coincidence may not occur for a substantial period of time. On the other hand, if the locally generated and received PRS chip rate are very different, the sequences come into phase quickly, but remain in phase for such a short period that the local oscillator PLL has very low effective loop gain, which results in slow acquisition, or in inability to acquire lock. Among the strategies for speeding up acquisition by varying the local chip rate to attempt to cause the locally generated and received PRS sequences to move into phase more quickly is a switch arrangement described in U.S. Pat. No. 4,319,358 issued Mar. 9, 1982, to Sepp, which selects an auxilary clock generator during acquisition.
Another strategy for fast acquisition involves incoherently converting the pseudorandom sequence encoded data IF signal to baseband with quadrature related local oscillator (LO) signals to produce inphase (I) and quadrature (Q) components of the baseband signal. If one of the I or Q components has a maximum value while the other has a zero value, the local oscillator is inphase with the IF carrier frequency. However, the local oscillator signal will generally be in some random phase relative to the IF carrier, and the I and Q components will have some finite magnitude less than the maximum possible value. The I and Q components of the received signal are applied to a pair of code matched filters which respond to the received PRS code to produce impulse signals at the time the received PRS code matches the filter code. The amplitude of the pulses produced by the two matched filters is related to the phase angle between the local oscillator signal and the IF carrier component of the received signal. These amplitudes are evaluated by a logic circuit to establish the phase error, and a control signal is generated which is applied to the local oscillator to perform a step correction of the phase, whereby the local oscillator frequency and phase become subhstantially identical or coherent with the frequency and phase of the IF carrier of the received signal. When this local oscillator signal is mixed with the IF signal, the desired baseband coded data (or its I and Q components) is generated. At this time, the output of one of the matched filters would be at a maximum amplitude and the output amplitude of the other matched filter will be substantially zero. The impulses produced by the matched filters may be processed to provide the desired data, or other known techniques may be used to extract the data from the baseband encoded data. Such an arrangement is described in U.S. patent application Ser. No. D 705,710 filed Feb. 26, 1985, now U.S. Pat. No. 4,538,048, patented in April 1986, in the name of Gumacos et al. These systems are suitable for burst communications at low carrier frequencies. When such systems are used at high carrier frequencies or in conjunction with moving platforms such as aircraft, satellites and the like, phase acquisition may be slow or may not occur at all because of the modulation of the incoherently demodulated I and Q signals by the difference frequency between the IF carrier and the demodulating signal. It would be desirable to have a CDMA spread spectrum communications system providing access to multiple transmitters and multiple receivers, operating in a burst mode in which the receivers rapidly select the desired signal by code identification of signals in the common bandwidth.